Fixed torch to turn it on when start scanning if torch mode is MTBTorchModeOn

[felipowsky]

I noticed this issue when I tried to start scanning with torch mode set to MTBTorchModeOn.
If I tried to do that it wouldn't work because hasExistingSession property was still NO.

You can reproduce this issue using the following code:

_scanner = [[MTBBarcodeScanner alloc] initWithPreviewView:_previewView];
_scanner.torchMode = MTBTorchModeOn;
[_scanner startScanningWithResultBlock:^(NSArray *codes) {
}];

[mikebuss]

Thanks - good catch!